The proposed studies will examine the mechanisms by which retinal projection neurons make precise and functional connections with their central targets. The hypothesis that neurotrophic factors modulate optic axon arborization, and synapse formation and stabilization will be tested in the live developing brain. Specifically, the cellular and molecular mechanisms by which the neurotrophin brain-derived neurotrophic factor (BDNF) controls axon and dendritic arborization, and synapse formation/stabilization will be examined in live, anesthetized tadpoles. The Xenopus laevis visual system offers a uniquely accessible vertebrate model in which the development of neuronal connections can be followed over time in the intact embryo. Embryologic manipulations and in vivo imaging techniques (time-lapse confocal microscopy and calcium imaging techniques) will be combined to follow, in real time, the morphological differentiation of pre- and post-synaptic partners and the formation of synaptic connections between the retina and its target optic tectum. By imaging individual optic axon terminal arbors and/or tectal neuron dendritic arbors while simultaneously visualizing synaptic sites in the intact animal, the following hypotheses will be tested: BDNF modulates activity-dependent competition between developing retinal inputs by promoting synapse stabilization between optic axons and target tectal neurons. Target-derived BDNF directly influences optic axon terminals to modulate axon arborization and synapse formation. The BDNF-elicited changes in optic axon arbor complexity lead to structural changes in tectal neuron synaptic complexity. BDNF induces localized changes in intracellular calcium levels in arborizing optic axons that are responsible for synapse stabilization and the formation of new branches. The results of these studies will provide valuable insights into fundamental mechanisms of synaptogenesis in the living brain, and will further our understanding of the mechanisms that control the development of visual pathways, that are critically important in the maintenance of normal visual function.